libagl: Add eglGetRenderBufferANDROID() extension

which returns the current render buffer as an android_native_buffer_t*

This was removed in AOSP for Gingerbread, but at CAF they put it back
as part of their patch for re-enabling the usage of copybit for
accelerating 2D rendering -> http://tinyurl.com/65mm5p6

Change-Id: I0165bf5b481ae097848c1c644785819f0c127ef1